The invention relates to a sparking plug for a combustion engine, comprising a metal shell with an externally threaded end part for screwing into a tapped hole in a cylinder head, the shell supporting, on the inside, an insulating body, particularly one made of ceramic, through which there passes a central electrode capable of being connected to a high tension, an annular chamber being delimited by an interior surface in the region of the threaded end past of the shell and the exterior surface of the end of the insulating body, the sparking plug also being equipped with at least one pressure sensor, particularly of the resistive type, and designed to subject this sensor to the pressure of an associated combustion chamber when the sparking plug is in place.
Measuring pressure in the cylinder of a combustion engine is of great importance given the subsequent thermodynamic calculations that can be made on the basis of the pressure cycle.
The precision with which pressure is measured, and the timing of the cycle are the key factors in calculating the energy released by the combustion.
It has already been proposed for such measurement to be performed using piezo-electric sensors based on quartz crystals. The quartz crystal is compressed via the pressure obtaining in the cylinder. The application of pressure to quartz generates an amount of current, expressed in picocoulombs, which has to be amplified in a charge amplifier. This solution is simultaneously expensive in terms of the sensor and the amplifier, and difficult to implement because a drift in the miniscule amount of current frequently occurs. This type of measurement is accompanied by a procedure and frequent checks of the hardware.
Furthermore, a piezo-electric sensor gives a good representation of the difference or variation in pressure applied to the sensor, but does not give the actual or absolute value of the pressure. Calibrating this offset is always a problem.
It has therefore been proposed that piezo-electric sensors be replaced with piezo-resistive sensors. The latter have two advantages:
they give the actual value of the absolute pressure,
the output signal is very easy to amplify, using a Wheatstone bridge.
By contrast, piezo-resistive sensors have the drawback of poor temperature resistance. However, it is currently possible to obtain piezo-resistive sensors which are capable of withstanding temperatures of the order of 250xc2x0 C., which is high enough for being installed in sparking plugs.
Various solutions for installing sensors, particularly piezo-resistive sensors, in sparking plugs have been proposed.
However, the installations hitherto proposed are not entirely satisfactory because they disturb the way combustion takes place and lead to measured pressure values which differ from the actual values. Certain installations involving a hole of somewhat sizeable volume for conveying the pressure onto the sensor, modify the volume of the chamber and give rise to resonant signals which are not consistent with the actual combustion phenomenon.
The object of the present invention is, mainly, to propose a way of installing the pressure sensor, particularly a piezo-resistive sensor, which avoids any appreciable modification to the way in which combustion takes place and which makes it possible to obtain measured pressure values which are as exact as possible, while at the same time adequately protecting the sensor against heat.
According to the invention, a sparking plug for a combustion engine, of the type defined hereinabove, equipped with a pressure sensor, is characterized in that the pressure sensor is installed in the threaded end part of the shell delimiting the annular chamber radially out from the internal surface of this end part such that the geometry of the annular chamber is not altered by the presence of the sensor.
As a preference, the threaded end part of the shell comprises, toward the interior, a recess for accommodating a barrel section capable of reproducing the geometry of the annular chamber; the barrel section comprises, on its external surface, a housing for the sensor which is thus enclosed in a closed space bounded by the barrel section and the wall of the recess, at least one small-diameter hole being provided in the wall of the barrel section to allow the pressure from the combustion chamber onto the sensor.
The enclosed space left for the sensor protects it against the temperature of the combustion chamber.
In an alternative form, the sensor is bonded to the external face of the housing of the barrel section and the hole opens into the said housing some distance from the sensor, so that the latter is subjected to the pressure of the combustion chamber on its opposite face to the face that is bonded to the barrel section.
In another alternative form, the sensor is bonded to the external face of the housing of the barrel section and the hole opens onto that face of the sensor which is bonded to the barrel section.
The barrel section may be bonded into the recess in the threaded end part of the shell. The sensor may be bonded into the housing in the barrel section.
In one particular embodiment, at least one ground electrode is connected to the shell and to ground, the tips of said electrodes being spaced apart by a predetermined distance. The barrel section may have at least one slot for the passage of the ground electrode(s).
The pressure sensor may be diametrically opposite the ground electrode.
The barrel section may carry a number of pressure sensors.
In another embodiment, the barrel section has at least one axial extension forming a ground electrode, the tips of the central electrode and of the ground electrode(s) being spaced apart by a predetermined distance.
Advantageously, at least one channel is pierced in the shell parallel to the axis of the sparking plug, and opens at one end into the closed space in which the sensor is housed and at its other end into a space connected to the outside, this channel allowing the passage of conducting wires to lead out the electrical signals from the pressure sensor.
At least one temperature sensor may also be provided on the barrel section.
The invention also relates to a combustion engine, the cylinder or cylinders of which is or are equipped with such sparking plugs.
The invention consists, apart from the provisions set out hereinabove, in a certain number of other provisions which will be explained with regard to two particular embodiments which are described in detail with reference to the drawings appended hereto but which are not in any way restrictive.